Coordinate input system using a tablet

ABSTRACT

A coordinate input system using an electronic tablet with four electrodes and four lines connected to the four electrodes. The four lines diverge into eight lines, and the eight lines are connected to a connector. Four of the eight lines reflect the electrical potential on the four electrodes. These four lines are connected to an A/D converter. The remaining four lines supply a voltage to the four electrodes. This coordinate system detects initial electrical potential values with hardware.

This is a continuation of application Ser. No. 08/814,510, filed Mar.10, 1997--all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coordinate input system using atablet provided for detecting an axial coordinate. The coordinate inputsystem using the tablet was designed for a small personal computer, andcan be carried with the computer.

2. Related Art

A coordinate input system using the tablet has an advantage in that itis able to combine a liquid crystal indication screen and a resistancemembrane tablet in one body.

A resistance membrane tablet of the coordinate input system has a filmside resistance membrane for the X coordinate input and a glass sideresistance membrane for the Y coordinate input. The film side resistancemembrane consists of transparent matter. An insulating material issandwiched between the film side resistance membrane and the glass sideresistance membrane. The insulating material breaks down under pressure.

When a stylus pen for indicating a coordinate touches the film sideresistance membrane, the film side resistance membrane and the glassside resistance membrane contact. As a result, a resistance divisionoccurs on both the transparent film side resistance membrane and theglass side resistance membrane. In this condition, electrodes on eachresistance membrane detect the electrical potential of each resistancemembrane.

The electrical potential detected by each electrode is converted into adigital signal by an A/D converter (analog-digital converter). CPUcalculates the X coordinate and the Y coordinate from this digitalsignal.

The known coordinate input system has four electrodes or a eightelectrodes. FIG. 6 shows a structure of a resistance membrane tablethaving four electrodes. FIG. 7 shows a structure of resistance membranetablet having eight electrodes. FIG. 8 shows an equivalent circuit ofthe resistance membrane tablet with four electrodes. FIG. 9 shows anequivalent circuit of the resistance membrane tablet with eightelectrodes.

The resistance membrane tablet with four electrodes has an input side10, four electrodes 11 (M1, M2, M3, M4), and four lines 12 (A1, A2, A3,A4). The resistance membrane tablet with eight electrodes has an inputside 10, eight electrodes 21 (M1, M2, M3, M4, N1, N2, N3, N4), and eightlines 22 (A1, A2, A3, A4, A5, A6, A7, A8). The area of the electrodepart of the tablet with eight electrodes is about 2 times the area ofthe electrode part of the tablet with four electrodes.

Next, the an operation of detecting of coordinates is explained for theresistance membrane tablet with four electrodes. When the stylus pen PEcontacts a point Pf on the input side 10, the film side resistancemembrane 30 (Resistance value is Rf) contacts the glass side resistancemembrane 31 (Resistance value is Rg). As a result a resistance divisionRf1, Rf2, Rg1 and Rg2 occurs. Contact resistance 70 (R1) and 71 (R2)occur at the connection points with the line 12 (A1, A2) and a connector44.

By closing switchs SW1 and SW2, the circuit containing the resistancedivision 70 (R1) and 71 (R2) and the film side resistance membrane 30(Rf) closes. Then it supplies a voltage V (+5v) to each electrode M1 andM2 at both ends of the film side resistance membrane 30 (Rf) through theline A1 and A2. A value of electrical potential of the electrode M1 isestablished to 5 volts, and a value of electrical potential of theelectrode M2 is established to 0 volt (ground). On the other hand, byopening switch SW3 and SW4, the circuit containing the resistancedivision 72 (R3) and 73 (R4) and the glass side resistance membrane 31(Rg) opens. Then it doesn't supply the voltage to each electrode M3 andM4, at both ends of the glass side resistance membrane 31 (Rg).

In this condition, a potential drop of the resistance division Rf1 andRf2 occurs at the film side resistance membrane 30. Then, a value ofelectric potential of a point Pf of the film side resistance membrane 30becomes Vf. Equation (1) expresses the electric potential of the pointPf of the film side resistance membrane 30.

    Vf=(Rf2+R1)/(Rf+R1+R2)×V                             (1)

    Rf=Rf1+Rf2                                                 (2)

Because an electric current does not spread in the glass side resistancemembrane 31, a potential drop of the resistance division Rg1 and Rg2does not occur at the glass side resistance membrane 31. Accordingly,the value of the electrical potential detected by the electrode M3 or M4of the glass side resistance membrane 31 is the same as the electricpotential of the point Pf of the transparent film side resistancemembrane 30. Both the electrical potential detected by the electrode M3and the electrical potential detected by the electrode M4 have the samevalue, the value of the electrical potential of the point Pf of the filmside resistance membrane 30. A signal of electrical potential of thepoint Pf of the film side resistance membrane 30 detected by theelectrode M3 or M4 is taken out from the line A3 or A4 which wasconnected to each electrode (M3, M4).

FIG. 2 shows a functional structure of the coordinate input system usinga tablet. The lines A3 and A4 of the glass side resistance membrane 31are connected to the A/D converter 2. The A/D converter 2 converts theelectrical potential of the point Pf of the film side resistancemembrane 30 which has analog quantities into a digital signal. A CPU3calculates the X coordinate from the digital signal from the A/Dconverter 2.

For example, a position of the electrode M1 has a starting point of Xcoordinate (X=0), and a position of electrode M2 has a terminal of Xcoordinate (X=1024). The intervals from electrode M1 to electrode M2 arethen divided into a value of 1024 dots. It is then assumed that a valueof the resistance division 70 and a value of the resistance division 71are 0 (R1=R2=0). In this case, when the stylus pen PE points at aposition of the electrode M1 in order to point at a position of X=0dots, electrical potential (Vf) of the point PE of the film sideresistance membrane 30 becomes V volts. This electrical potential of thepoint PE of the film side resistance membrane 30 is detected by theelectrode M3 or M4. An expression of electrical potential of a point PEis shown with the next expression.

    Vf=(Rf+0)/(Rf+0+0)×V=V                               (3)

    Rf2=Rf                                                     (4)

The CPU3 calculates a value of X coordinate from the electricalpotential of point PE of the film side resistance membrane 30. As aresult CPU3 detects that X coordinate is X=0.

On the other hand, when the stylus pen PE points at a position of theelectrode M2 in order to point at a position of X=1024 dots, electricalpotential (Vf) of the point PE of the film side resistance membrane 30becomes 0 volts. This electrical potential of the point PE of the filmside resistance membrane 30 is detected by the electrode M3 or M4. Inthis case, the expression of electrical potential of a point PE is shownwith the next expression.

    Vf=(0+0)/(Rf+0+0)×V=0                                (5)

    Rf2=0                                                      (6)

The CPU3 calculates a value of the X coordinate from the electricalpotential of point PE and detects that X coordinate as X=1024.

When the stylus pen PE points at an intermediate point of electrode M1and electrode M2 in order to point at a position of X=512 dots,electrical potential (Vf) of the point PE of the film side resistancemembrane 30 becomes 1/2V volts. This electrical potential of the pointPE of the film side resistance membrane 30 is detected by the electrodeM3 or M4. In this case, the expression of electrical potential of apoint PE is shown with the next expression.

    Vf=(1/2Rf+0)/(Rf+0+0)×V=1/2V                         (7)

    Rf2=1/2Rf                                                  (8)

The CPU3 calculates a value of X coordinate from the electricalpotential of point PE and detects that X coordinate as X=512.

In case of detecting Y coordinate, by closing the switch SW3 and SW4,the circuit containing the resistance division 72 (R3) and 73 (R4) andthe glass side resistance membrane 31 (Rg) closes. Then it suppliesvoltage V (+5v) to each electrode M3 and M4 at the ends of the glassside resistance membrane 31. By opening the switch SW1 and SW2, thecircuit containing the resistance division 70 (R1) and 71 (R2), and thefilm side resistance membrane 30 (Rf) a opens. And a value of electricalpotential of the electrode M3 is established to 5 volts, and a value ofelectrical potential of the electrode M4 is established to 0 volt(ground). In this condition, a resistance division Rg1 and Rg2 occurs ata point Pg of the glass side resistance membrane 31. The potential dropof the resistance division Rg1 and Rg2 occurs at the glass sideresistance membrane 31, then a value of electric potential of the pointPg of the glass side resistance membrane 31 becomes Vg. An expression(9) shows an expression of the electric potential (Vg) of the point Pgof the glass side resistance membrane 31.

    Vg=(Rg2+R3)/(Rg+R3+R4)×V                             (9)

    Rg=Rg1+Rg2                                                 (10)

This electrical potential of the point Pg of the glass side resistancemembrane 31 is detected by the electrode M1 or M2. A signal ofelectrical potential of the point Pg of the glass side resistancemembrane 31 detected by the electrode M1 or M2 is taken out from theline A1 or A2 which was connected to each electrode (M1, M2)

Referring to FIG. 2, the lines A1 and A2 of the film side resistancemembrane 30 are connected with an A/D converter 2. The A/D converters 2convert the analogs electric potential of the point Pg of the glass sideresistance membrane 31, into a digital signal corresponding to the Ycoordinate. The CPU3 calculates the Y coordinate from the digital signalfrom the A/D converter 2. For example, a position of electrode M3 has astarting point of Y coordinate (Y=0) and electrode M4 has a terminal ofY coordinate (Y=1024). The intervals from electrode M3 to electrode M4into a value of 1024 dots.

The basic operation of coordinate detection resistance membrane tabletwith four electrodes is the same as mentioned above. But actually, it isneeded to consider the resistance division 70 (R1) and 71 (R2) of thefilm side resistance membrane 30 when detecting an X coordinate, and itis needed to consider the resistance division 72 (R3) and 73 (R4) of theglass side resistance membrane 31 when detecting a Y coordinate. Inaddition, the value of each resistance division (70,71,72,72) changesaccording to the temperature change and a change of contact state ofbetween each line (A1,A2,A3,A4) and the connector 44.

For example, a potential drop of 0.5 volts occurs by the contactresistance 70 of the line A1 connected to the electrode M1, and apotential drop of 0.5 volts occurred by the contact resistance 71 of theline A2 connected to the electrode M2. In this state, the value ofelectrical potential of the electrode M1 is 4.5 volts, and the value ofelectrical potential of the electrode M2 is 0.5 volts. Accordingly adifference of electrical potential between the electrode M1 and theelectrode M2 becomes 4.0 volts. In this case, when electrical currentpotential Vf=4.5 volts was detected, the CPU3 needs to calculate the Xcoordinate as X=0. When an electrical current potential Vf=0.5 volts wasdetected, the CPU3 needs to calculate the X coordinate as X=1024. And byrange of difference 4.0 volts of electrical potential with the electrodeM1 and the electrode M2, the CPU3 needs to assign a range of Xcoordinates of 1024 dots.

The next expression (11) shows the difference in electrical potential ofthe electrodes M1 and M2.

    Vfv=Rf/(Rf+R1+R2 )×V                                 (11)

The next time, each contact resistance of lines A1 and A2 changes, as aresult a potential drop of 0.7 volts occurs by the contact resistance 70and a potential drop of 0.8 volts occurs by the contact resistance 71.By this state, a value of electrical potential of the electrode M1 is4.3 volts, and a value of electrical potential of the electrode M2 is0.8 volts. Accordingly a difference of electrical potential between theelectrode M1 and the electrode M2 becomes 3.5 volts. In this case, whenan electrical potential Vf=4.3 volts was detected, the CPU3 needs tocalculate the X coordinate as X=0. When an electrical current potentialVf=0.7 volts was detected, the CPU3 needs to calculate the X coordinateas X=1024. Using the difference of 3.5 volts between the electrode M1and the electrode M2, the CPU3 assigns a range of X coordinates of 1024dots.

For the detection of Y coordinates, the CPU3 needs to consider thecontact resistance of the electrode M3 and the electrode M4. The CPU3needs to calculate an X coordinate with X=1024. Using the difference ofelectrical potential between electrodes M3 and M4, the CPU3 assigns arange of X coordinate of 1024 dots. Accordingly, before detecting an Xcoordinate and a Y coordinate, the CPU3 detects the value of electricalpotential of each electrode as having given the voltage V.

However, in case of the coordinate input system of resistance membranetablet of four electrodes, the value of electrical potential of eachelectrode (M1, M2, M3, M4) is taken out from each line (A1, A2, A3, A4)connected to each electrode and having the contact resistance (R1, R2,R3, R4). Accordingly, it can't detect the electrical potential of eachelectrode precisely because it can't detect the potential drop of eachline with this hardware. The coordinate input system with a resistancemembrane tablet of four electrodes must detect each potential drop ofeach line with software.

Next, a coordinate detection operation is explained for the coordinateinputting system with resistance membrane tablet of eight electrodes.FIG. 7 shows a structure of a resistance membrane tablet of thecoordinate inputting system with a resistance membrane tablet of eightelectrodes. The resistance membrane tablet with eight electrodes detectsthe potential drop of each line with hardware. Accordingly, it candetect the electrical potential of each electrode with hardwareprecisely.

FIG. 9 shows an equivalent circuit of the resistance membrane tabletwith eight electrodes. The film side resistance membrane 30 includes, inaddition to the electrodes M1 and M2 for the voltage V supply,electrodes N1 and N2 for electrical potential detection. The glass sideresistance membrane 31 includes, in addition to the electrode M3 and M4for the voltage V supply, electrodes N3 and N4 for electrical potentialdetection.

Next, a coordinate detection operation is explained for the coordinateinputting system using a resistance membrane tablet with eightelectrodes. When it detects the X coordinate, the switches SW1 and SW2are closed. In this condition, the circuit containing the resistancedivision 70 (R1) and 71 (R2) and the film side resistance membrane 30(Rf) is closed. Then it supplies a voltage V to each electrode M1 and M2at the ends of the film side resistance membrane 30 (Rf) through linesA1 and A4. When the stylus pen PE comes contacts the point Pf of thefilm side resistance membrane 30, the electrical potential of the pointPf is detected by the electrodes N3 and N4. Both electrical potentialsdetected by the electrodes N3 and N4 are equal. The electrical potentialdetected with electrode N3 or N4 is taken out from a line A6 or A7, andis output to the A/D converter 2.

When it detects the Y coordinate, the switches SW3 and SW4 are closed.In this condition, the circuit containing the resistance division 72(R3) and 73 (R4), and the glass side resistance membrane 31 (Rg) isclosed. Then it supplies a voltage V to each electrode M3 and M4 at theends of the glass side resistance membrane 31 (Rg) through lines A5 andA8. When the stylus pen PE contacts the point Rf of the film sideresistance membrane 30, the electrical potential of the point Pg isdetected by the electrodes N1 and N2. Both electrical potentialsdetected by the electrodes N1 and N2 are equal. The electrical potentialdetected with the electrode N1 or N2 is taken out from a line A2 or A3,and is output to the A/D converter 2.

Next, an operation to detect an initial value of the electricalpotential of each electrode (M1, M2, M3, M4) with a hardware isexplained. When it detects, the electrical potentials of the electrodesN1 and the electrode N2, the switches SW1 and SW2 are closed, and itsupplies a voltage V to each electrode M1 and M2 through lines A1 andA4. The electrical potential of the electrode N1 is detected on the lineA2. The electrical potential on the electrode N2 is detected on the lineA3. The electric current does not flow in the line A2 and the line A3.Accordingly, because the voltage drop from contact resistance 80 andcontact resistance 81 does not occur, the electrical potential on eachline A2 and A3 is almost the same as the electrical potential of eachelectrode N1 and N2. When it detects the electrical potential of theelectrodes N3 and N4, the switches SW3 and SW4 are closed, and a voltageV appears at each electrode M3 and M4. The electrical potential of theelectrode N3 is detected on the line A6. The electrical potential of theelectrode N4 is detected on the line A7. The electric current does notflow in the lines A6 and A7. Accordingly, because the voltage drop fromcontact resistance 82 and contact resistance 83 does not occur, theelectrical potential on each line A6 and A7 is almost the same as theelectrical potential of each electrode N3 and N4.

In this way, the resistance membrane with eight electrodes detects eachpotential drop of each line with hardware. But the area of the electrodepart of the tablet with eight electrodes is about 2 times the area ofthe electrode part of the tablet with four electrodes. The coordinateinput system with four electrodes can not detect each potential drop ofeach line with hardware. Accordingly, when the voltage drop of a linechanges because of change of contact resistance, it can not detectaccurately the voltage drop with hardware. The CPU2 then can notcalculate the value of the X and Y coordinates precisely.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved coordinate input system. An improved tablet detects an initialvalue of electrical potential of each electrode with hardware with onlyfour electrode.

In accordance with the present invention, the foregoing objects, amongothers, are achieved by providing a coordinate input system uses atablet comprising a film side resistance membrane to detect the Xcoordinate, a glass side resistance membrane to detect the Y coordinate,a pair of film side electrodes on the film side resistance membrane, apair of glass side electrodes on the glass side resistance membrane, apair of film side lines connected to the pair of film side electrodes asone line and then diverged into two lines, and a pair of glass sidelines connected to the pair of glass side electrodes as one line andthen diverged into two lines.

There has also been provided, in accordance with yet another aspect ofthe present invention, a coordinate input system which using a tabletcomprising a film side resistance membrane to detect the X coordinate, aglass side resistance membrane to detect the Y coordinate, a pair offilm side electrodes on the film side resistance membrane, a pair ofglass side electrodes on the glass side resistance membrane, a pair offilm side lines connected to the pair of film side electrodes, and apair of glass side lines, which are connected to each the pair of glassside electrodes, and a connector connecting the pair of film side linesto the pair of glass side lines at two points.

There has also been provided, in accordance with yet another aspect ofthe present invention, a coordinate input system using tablet comprisinga film side resistance membrane to detect the X coordinate, a glass sideresistance membrane to detect the Y coordinate, a pair of film sideelectrodes on said glass side resistance membrane to detect theelectrical potential corresponding to the Y coordinate of a pointpointed at by a stylus pen, a pair of glass side electrodes on said filmside resistance membrane to detect the electrical potentialcorresponding to the X coordinate of the point, and a pair of film sidelines connected to the pair of film side electrodes as one line then anddiverged into one line to give the voltage and one line to take out asignal of electrical potential detected by the pair of film sideelectrodes, a pair of glass side lines connected to the pair of glassside electrodes as one line and then diverged into one line to give thevoltage and one line to take out a signal of electrical potentialdetected with the pair of glass side electrodes, means for supplying avoltage to the pair of film side electrodes and the pair of glass sideelectrodes, an A/D converter for converting a signal of electricalpotential detected with the pair of film side electrodes and the pair ofglass side electrodes into a digital signal, and a CPU for calculatingthe X coordinate from the digital signal detected by the pair of glassside electrodes, and for calculating the Y coordinate from the digitalsignal detected by the pair of film side electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of the resistance membrane tablet of thecoordinate input system according to this invention.

FIG. 2 shows the functional structure of the coordinate input system.

FIG. 3 shows an equivalent circuit of a resistance tablet of thecoordinate input system according to this invention.

FIG. 4 shows a second embodiment of a resistance membrane tablet of thecoordinate input system according to this invention.

FIG. 5 shows a third embodiment of a resistance membrane tablet of thecoordinate input system according to this invention.

FIG. 6 shows an embodiment of a resistance membrane tablet of thecoordinate input system with four electrodes.

FIG. 7 shows an embodiment of a resistance membrane tablet of acoordinate input system with eight electrodes.

FIG. 8 shows an equivalent circuit of the resistance membrane tabletwith four electrodes.

FIG. 9 shows an equivalent circuit of the resistance membrane tabletwith eight electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A resistance membrane tablet 1 of this invention shown in FIG. 1comprises an input side 10, four electrodes 11 (M1, M2, M3, M4), andeight lines 22 (A1, A2, A3, A4, A5, A6, A7, A8). The electrodes M1 andM2 detect the Y coordinate. The electrodes M1 and M2 are on oppositesides of a film side resistance membrane 30. The electrodes M3 and M4detect the X coordinate. The electrodes M3 and M4 are on opposite sidesof a glass side resistance membrane 31. Each electrode 11 (M1, M2, M3,M4) has the same function as the electrodes of the coordinate inputsystem with four electrodes. The four lines 22 (A2, A3, A6, A7) reflectthe electrical potential on the each electrodes 11. A connector 42connects these four lines to an A/D converter 2. The four lines (A1, A4,A5, A8) supply a voltage to the electrodes 11. Before connecting withconnector 42, the lines 22 diverge into eight lines, and those eightlines are connected to connector 42.

When the stylus pen PE contacts the film side resistance membrane 30,the electrode M1 or M2 detects an electrical potential of a point Pg onthe glass side resistance membrane 31 corresponding to the Y coordinate.The electrode M3 or M4 detects an electrical potential of a point Pf onthe film side resistance membrane 30 corresponding to the X coordinate.The electrical potential, corresponding to the X coordinate, on theelectrode M1 or M2 is converted into a digital signal by an A/Dconverter 2. The electrical potential, corresponding to the Ycoordinate, on the electrode M3 or M4 is also converted into digitalsignal by the A/D converter 2. The CPU3 calculates the X coordinate fromthe digital signal corresponding to the X coordinate. The CPU3 alsocalculates the Y coordinate from the digital signal corresponding to theY coordinate. Each connection between connector 42 and lines (A1, A2,A3, A4) creates a contact resistance (70, 90, 91, 71). Each connectionbetween connector 42 and lines (A5, A6, A7, A8) also creates a contactresistance (72, 92, 93, 73).

The line A1 connects to the line A2 at a point P. A single line thenconnects the lines A1 and A2 to the electrode M1 of the film sideresistance membrane 30. The line A1 delivers a voltage V (+5V) to filmside resistance membrane 30 and the electrode M1. The line A2 detectsthe electrical potential Vg of a point Pg of the glass side resistancemembrane 31 when the Y coordinate is detected. The line A2 also detectsthe electrical potential of the electrode M1 when the stylus pen PE isnot applied. The line A2 thus detects the voltage drop caused by contactresistance 70 (R1) of line A1. The line A3 connects to the line A4 at apoint P2. A single line then connects the line A3 and the line A4 to theelectrode M2 of the film side resistance membrane 30. The line A4delivers voltage V (ground) to the film side resistance membrane 30 andthe electrode M2. The line A3 detects the electrical potential Vg of thepoint Pg of the glass side resistance membrane 31 when the Y coordinateis detected. The line A3 also detects the electrical potential of theelectrode M2 when the stylus pen PE is not applied. The line A3 thusdetects the voltage drop caused by contact resistance 71 (R2) of lineA4.

The line A5 connects to the line A6 at a point P3. A single line thenconnects the line A5 and the line A6 to the electrode M3 of the grassside resistance membrane 31. The line A5 delivers a voltage V (+5V) tothe glass side resistance membrane 31 and the electrode M3. The line A6detects the electrical potential Vf of a point Pf of the film sideresistance membrane 30 when the X coordinate is detected. The line A6also detects electrical potential of the electrode M3 when the styluspen PE is not applied. The line A6 thus detects the voltage drop by thecontact resistance 72 (R3) of line A5. The line A7 connects to the lineA8 at a point P4. A single line then connects the line A7 and the lineA8 to the electrode M4 of the glass side resistance membrane 31. Theline A8 delivers a voltage V (ground) to the glass side resistancemembrane 31 and the electrode M4. The line A7 detects the electricalpotential Vf of the point Pf of the film side resistance membrane 30when the X coordinate is detected. The line A7 also detects theelectrical potential of the electrode M4 when the stylus pen PE is notapplied. Thus, the line A7 detects the voltage drop by contactresistance 73 of line A8 (R4).

Now, an operation to detect the X coordinate and the Y coordinate isexplained. The detection is similar to the detection in the resistancemembrane tablet of four electrodes.

When the stylus pen PE contacts the point Pf of the film side resistancemembrane 30, the film side resistance membrane 30 contacts the glassside resistance membrane 31. As a result, a resistance division Rf1 andRf2 is generated in the film resistance membrane side 30, and aresistance division Rg1 and Rg2 is generated in the glass sideresistance membrane 31.

First, the detection of the X coordinate is explained. By closing aswitches SW1 and SW2, the circuit containing the contact resistance 70(R1) and 71 (R2), and the resistance division Rf1 and Rf2 closes. Thenthe voltage V (5 volts) is delivered to the film side resistancemembrane 30 from the electrode M1 and the electrode M2 through lines A1and A4. The circuit containing the contact resistance 72 (R3) and 73(R4), and the resistance division Rg1 and Rg2 opens. Then the voltage V(5 volts) is not delivered to the glass side resistance membrane 31. Ifthe circuit of the film side resistance membrane 30 closes, a voltagedrop occurs because of the resistance division Rf1 and Rf2. As a result,the electrical potential at the point Pf of the film side resistancemembrane 30 becomes Vf. This electrical potential Vf corresponds to theX coordinate as shown in expression (1). This electrical potential Vf ofthe point Pf of the film side resistance membrane 30 is detected by theelectrode M3 or M4. The signal of the electrical potential Vf of thepoint Pf is taken out by the line A6 and A7 of the glass side resistancemembrane 31. The A/D converter 2 converts the electrical potential intoa digital signal corresponding to the X coordinate.

Next, the operation of detection of the Y coordinate is explained. Byclosing the switches SW1 and SW2, the circuit containing the contactresistance 72 (R3) and 73 (R4), and the resistance division Rg1 and Rg2closes. Then, the voltage V (5 volts) is delivered to the glass sideresistance membrane 31 from the electrode M3 and the electrode M4through lines A5 and A8. The circuit containing the contact resistance70 (R1) and 71 (R2), and the resistance division Rf1 and Rf2 opens.Thus, the voltage V (5 volts) is not delivered to the film sideresistance membrane 30. If the circuit of the glass side resistancemembrane 31 closes, a voltage drop occurs because of the resistancedivision Rg1 and Rg2. As a result, the electrical potential of the pointPg of the glass side resistance membrane 31 becomes the electricalpotential Vg. This electrical potential Vg of the point Pg of the glassside resistance membrane 31 is detected by the electrode M1 or M2. Asignal of the electrical potential Vg is taken out by line A2 or A3 ofthe film side resistance membrane 30. The A/D converter 2 converts thiselectrical potential into a digital signal corresponding to the Ycoordinate.

Next, an operation to detect an initial value of electrical potential ofeach electrode with the hardware of this embodiment will be explained.The operation of detecting the initial value of electrical potential ofeach electrode is similar to the operation of the coordinate inputsystem with eight electrodes. First, an operation to detect an initialvalue of the electrical potential of the electrodes M1 and M2 of thefilm side resistance membrane 30 is explained. When the stylus pen PEdoes not contact the film side resistance membrane 30, a voltage V (5volts) appears on each electrode M1 and M2 from lines A1 and A4. Theinitial value of the electrical potential of the electrodes M1 and M2 isthen detected. The initial value of the electrical potential on theelectrodes M1 is detected, and the initial value of electrical potentialof the electrode M1 is reflected on the line A2. The initial value ofthe electrical potential on the electrode M2 is also detected, and theinitial electrical potential on the electrode M2 is reflected on theline A3. The lines A2 and A3 have contact resistance 90 and 91. Becausean electric current does not flow in the lines A2 and A3, a voltage dropacross contact resistance 90 and 91 does not occur. Accordingly, thecontact resistance 90 and 91 can be ignored. On this account, each lineA2 and A3 reflects the precise value of the electrical potential of theelectrodes M1 and M2. For example, the position of the electrode M1 isthe starting point of the X coordinate, and the position of theelectrode M2 is a terminal of the X coordinate. The intervals fromelectrodes M1 to M2 are divided into a values of 1024 dots. The initialvalue of the electrical potential on the electrode M1 is detected as theelectrical potential of the point Pf when the stylus pen PE points to aposition X=0 dots. The initial value of the electrical potential of theelectrode M2 is detected as the electrical potential of the point Pfwhen the stylus pen PE points to a position X=1024 dots.

Next, An operation to detect an initial value of the electricalpotential of the electrodes M3 and M4 of the glass side resistancemembrane 31 is explained. If the stylus pen PE does not contact the filmside resistance membrane 30, the voltage V (5 volts) appears on theglass side resistance membrane 31 from electrodes M3 and M4 throughlines A5 and A8. The initial value of the electrical potential of theelectrodes M3 and M4 is then detected. The initial value of theelectrical potential of the electrode M3 is detected, and the initialvalue of the electrical potential of the electrode M3 is reflected onthe line A6. The initial value of the electrical potential of theelectrode M4 is also detected, and the initial value of the electricalpotential of the electrode M4 is reflected on the line A7. The lines A6and A7 have the contact resistance 92 and 93. Because an electriccurrent does not flow in each line the A6 and A7, a voltage drop acrossthe contact resistance 92 and 93 does not occur. Accordingly, thecontact resistance 92 and 93 can be ignored. On this account, each lineA6 and A7 reflects the precise value of the initial electrical potentialof each electrodes M3 and M4. For example, a position of the electrodeM3 is the starting point of the Y coordinate, and a position of theelectrode M4 is a terminal of the Y coordinate. The interval fromelectrode M3 to electrode M4 into a value of 1024 dots. The initialvalue of the electrical potential of the electrode M3 is detected as theelectrical potential of the point Pg when the stylus pen PE points to aposition Y=0 dots. The initial value of the electrical potential of theelectrode M4 which detected it is established as electrical potential ofthe point Pg when the stylus pen PE points to a position Y=1024 dots.According to above operation, an initial value of electrical potentialinfluenced by the contact resistance of each electrode is detected withhardware.

FIG. 4 shows a second embodiment of a resistance membrane tablet for acoordinate input system. In The resistance membrane tablet shown in FIG.4, the shape of lines 22 is modified. Lines 22 connect to electrodes 11and diverge at a position near the connector 40.

FIG. 5 shows a third embodiment of a resistance membrane tablet for acoordinate input system. In the resistance membrane tablet shown in FIG.5, the shape of lines 43 is modified. The line width of lines 43increases at the connector 42. Thus, two terminals of the connector 42(a terminal to detect electrical potential and a terminal to supply thevoltage) connected to one line.

What is claimed is:
 1. A tablet device for use with an electronic inputcoordinate system, comprising:a first resistance membrane to detect afirst coordinate; a second resistance membrane to detect a secondcoordinate; said first resistance membrane overlying said secondresistance membrane; a first electrode coupled to said first resistancemembrane; a second electrode coupled to said second resistance membrane;a first line coupled to said first electrode at one point and thereafterdiverging into two lines including a first voltage supply line to supplyvoltage to said first resistance membrane and a first electricalpotential monitor line to monitor an electrical potential of said firstelectrode; a second line coupled to said second electrode at one pointand thereafter diverging into two lines including a second voltagesupply line to supply voltage to said second resistance membrane and asecond electrical potential monitor line to monitor an electricalpotential of said second electrode; wherein when said first electricalpotential monitor line of said first line monitors said electricalpotential of said first electrode to detect an initial value ofelectrical potential of said first electrode, said first voltage supplyline of said first line supplies voltage to said first resistancemembrane; when said second electrical potential monitor line of saidsecond line monitors said electrical potential of said second electrodeto detect an initial value of electrical potential of said secondelectrode, said second voltage supply line of said second line suppliesvoltage to said second resistance membrane; when said first electricalpotential monitor line of said first line monitors said electricalpotential of said first electrode to detect said second coordinate, saidsecond voltage supply line of said second line supplies voltage to saidsecond resistance membrane; and when said second electrical potentialmonitor line of said second line monitors said electrical potential ofsaid second electrode to detect said first coordinate, said firstvoltage supply line of said first line supplies voltage to said firstresistance membrane.
 2. The tablet device for use with an electronicinput coordinate system according to claim 1, wherein said first linediverges into two lines at a position near a connector; andsaid secondline diverges into two lines at a position near said connector.
 3. Thetablet device for use with an electronic input coordinate systemaccording to claim 1, wherein said first line and said second line eachdiverging into two lines at respective positions near each other.
 4. Atablet device for use with an electronic input coordinate system,comprising:a first resistance membrane to detect a first coordinate; asecond resistance membrane to detect a second coordinate; said firstresistance membrane overlying said second resistance membrane; a firstelectrode coupled to said first resistance membrane; a second electrodecoupled to said second resistance membrane; a first line coupled to saidfirst electrode; a second line coupled to said second electrode; aconnector coupled to said first line at two coupling points including afirst voltage supply point to supply voltage to said first resistancemembrane and a first electrical potential monitor point to monitor anelectrical potential of said first electrode, and coupled to said secondline at two coupling points including a second voltage supply point tosupply voltage to said second resistance membrane and a secondelectrical potential monitor point to monitor an electrical potential ofsaid second electrode; wherein when said connector monitors saidelectrical potential of said first electrode to detect an initial valueof electrical potential of said first electrode by said first electricalpotential monitor point, said connector supplies voltage to said firstresistance membrane by said first voltage supply point; when saidconnector monitors said electrical potential of said second electrode todetect an initial value of electrical potential of said second electrodeby said second electrical potential monitor point, said connectorsupplies voltage to said second resistance membrane by said secondvoltage supply point; when said connector monitors said electricalpotential of said first electrode to detect said second coordinate bysaid first electrical potential monitor point, said connector suppliesvoltage to said second resistance membrane by said second voltage supplypoint; and when said connector monitors said electrical potential ofsaid second electrode to detect said first coordinate by said secondelectrical potential monitor point, said connector supplies voltage tosaid first resistance membrane by said first voltage supply point. 5.The tablet device for use with an electronic input coordinate systemaccording to claim 4, wherein said connector has pairs of terminals.